[MRG] DOC fix layout: ensure stable width of documentwrapper

Fixes bugs described at https://github.com/scikit-learn/scikit-learn/commit/937f36cba77f18af7b37f0798f76aaeaed0b02d8#commitcomment-8720925

I've only tested this by looking at BallTree (which is affected particularly because of its lack of content) and a class with examples. Both issues seem to be fixed (and I have no idea why I originally made those CSS changes).